Typically, there are three main techniques to achieve an extended depth of field (hereinafter referred to as EDOF). The first technique (See Non Patent Literature 1) employs an optical element, referred to as a phase plate, inserted in the optical system in order to make a blur uniform in the scene depth direction. Then, the technique executes image-restoration processing on an image obtained through the blur uniformity, using a previously-measured blur pattern or a calculated blur pattern based on a simulation. Hence, the technique generates an EDOF image. This technique is introduced as the wave-front coding (hereinafter referred to as the WFC).
The second technique (See Non Patent Literature 2) employs an aperture of which pattern is modified, so that the distance to the focal plane is accurately measured for each of subregions of the image. Then, the technique executes image-restoration processing on each subregion, using a blur pattern which is based on each of previously-measured distances to a corresponding one of the subregions. Hence, the technique generates an EDOF image. This technique is introduced as the coded aperture (hereinafter referred to as the CA).
The third technique (See Non Patent Literature 3) involves shifting a focus lens or an imaging device during the exposure time in order to convolve images which are uniformly focused in the scene depth direction (in other words, obtaining a uniform blur in the scene depth direction). Then, the technique executes image-restoration processing on the image obtained through convolution, using a previously-measured blur pattern or a calculated blur pattern based on a simulation. Hence, the technique generates an EDOF image. This technique is introduced as the Flexible DOF (hereinafter referred to as the F-DOF).
There are other techniques than the above techniques. One of in the techniques (Non Patent Literature 4) involves estimating the depth and detecting the sharpness of the image, taking advantage of the on-axis chromatic aberration, and generating an all-focus image with image processing. Another technique (Non Patent Literature 5) involves making a uniform blur in the scene depth direction using a multifocal lens, and executing image-restoration processing on the image obtained through the uniformity using a previously-measured blur pattern or a calculated blur pattern based on a simulation. Compared with the first three techniques, however, the next two techniques fail to achieve as large an EDOF as the three techniques do.
In addition, there has been another technique referred to as the focal stack. This technique involves obtaining images each having a different focal point (focal position), and extracting a region-to-be-focused from each of the images. Then, the technique composes the extracted images to generate an EDOF image. Unfortunately, the technique requires many images to be obtained. Thus, the technique inevitably needs a relatively-long time period for obtaining the images, and occupies too much memory.
Various kinds of phase plates are proposed for one of the first three techniques, the WFC. Among the phase plates, the cubic phase mask (hereinafter referred to as the CPM) and the free-form phase mask (hereinafter referred to as the FPM) are introduced as the phase plates for obtaining the largest EDOF. In view of the image quality of the restored image (fewer artifacts), the FPM is more promising than the CPM (Non Patent Literature 6). As a weakness of the WFC, however, the phase plate inserted in the optical system tends to deteriorate the off-axis performance of the lens (Non Patent Literature 7). Specifically, the WFC cannot obtain as much a uniform blur with respect to incident light coming from other than the front as a uniform blur with respect to incident light coming from the front. As a result, when an image is restored with a use of an on-axis blur pattern, the off-axis quality of the restored image inevitably deteriorates.
The second technique among the first three techniques; namely the CA, employs an aperture having a modified pattern in order to increase the accuracy of the distance measurement. Due to the modified pattern inherent in the aperture of the technique, specific frequency components are lost from an obtained image and a restored image. In other words, the technique suffers image deterioration. Furthermore, the technique is not suitable for imaging in the dark since an amount of received light in the technique is typically less than that in an ordinary technique no matter how the shape of the aperture is to be modified.
The third of the first three techniques, the F-DOF, enjoys the most excellent image quality among all the three techniques, and achieves a large EDOF. The off-axis performance depends on the performance of the lens itself, which makes it easy to enhance the performance of the imaging apparatus. As an optical condition, however, the technique needs to employ an image-space telecentric lens since the same object needs to be convolved on the same position of the image even though the focal point shifts during the exposure.
The oldest application of the above EDOF technique is the one to microscopes. In the case of a microscope, the focal stack technique has long been used because a user can take time to obtain an image of a still object. The focal stack technique, however, requires much time and work as described above. Hence the EDOF has been disclosed in some references along with the F-DOF technique (Patent Literatures 1 to 4). When the F-DOF is used for the microscope, disclosed are the cases where, during the exposure, (i) a specimen; namely the object, is moved and (ii) the microscope tube is moved. Based on the premise of image-restoration processing after the exposure, it is reasonable to control the move such that a uniform blur is formed at all times on the object, since an image-restoration processing technique employing a single blur pattern is available (Patent Literature 5). In order to control the move, the object to be to moved should be moved at a constant speed in the case where the object is the imaging device. In the case where the focus lens is moved, the focus needs to be shifted as fast as the image plane shifting at a constant speed (Non Patent Literature 3). It is noted that the focus lens may be shifted from the far-end focal point to the near-end focal point and visa versa.
Recently, the EDOF technique has also been applied to a camera for cellular phones. The use of the EDOF technique for the camera contributes to making the camera smaller. In other words, the EDOF successfully obtains an all-focus image (all the objects in the image are focused) without an autofocus system.
In view of the application of the above techniques, the F-DOF itself is not adopted since the F-DOF requires a mechanism to shift the focus lens or the imaging device. Hence adopted is the WFC or the technique utilizing the on-axis chromatic aberration.
Another application to be considered is the one to regular digital still cameras and digital video cameras. In recent years, users have been looking for more user-friendly and further foolproof digital still cameras and digital video cameras. The EDOF technique is promising since the technique achieves an all-focus image, freeing a user from obtaining an out-of-focus image. In view of the application, the most excellent technique of all of the above techniques is the F-DOF since the F-DOF has the following features: (i) high image quality is available, (ii) the EDOF effect and the range of focus can be changed at the user's option, (iii) the technique is feasible with the application of a regular autofocus mechanism (no special optical system is required), and (iv) the user can easily switch between EDOF shooting and the regular shooting.
FIGS. 1 and 2 show structures required to achieve the F-DOF. FIG. 1 shows a structure of an imaging apparatus 500 which shifts a focus lens during the exposure period. The imaging apparatus 500 in FIG. 1 includes an imaging device 1, a lens 2, a shutter 3, a focus lens shift control unit 4, a shutter operation instructing unit 5, a release receiving unit 6, a focus lens initial position detecting unit 7, an exposure time determining unit 8, a focus lens position resetting unit 18, a synchronization managing unit 10, an image-restoration processing unit 11, a PSF storage unit 12, and an image data recording unit 13. Moreover, the lens 2 includes a focus lens 20 and a group of lenses other than the focus lens 20.
When the release receiving unit 6 receives an exposure start instruction from a user, the focus lens initial position detecting unit 7 detects the position of the focus lens 20 at that time (the initial position). Once the initial position is detected, the focus lens position resetting unit 18 shifts the focus lens 20 to a predetermined end position, such as the nearest end or the farthest end. Here, in a predetermined range of focus and with respect to the imaging apparatus 500, the nearest end is at the nearest distance to the imaging apparatus 500, and the farthest end is at the farthest distance from the imaging apparatus 500.
The focus lens position resetting unit 18 resets the focus lens 20, and at the same time, the exposure time determining unit 8 determines capturing parameters including a shutter speed and an f-number. As soon as the above operations end, the synchronization managing unit 10 gives an instruction to start exposure to the focus lens shift control unit 4 and the shutter operation instructing unit 5. Simultaneously, the synchronization managing unit 10 gives an instruction to the focus lens shift control unit 4 so that, within the exposure period and based on the end point of the focus lens 20 reset by the focus lens position resetting unit 18, the focus lens shift control unit 4 shifts the focus lens 20 (i) from the nearest end to the farthest end when the end point is at the nearest end, and (ii) from the farthest end to the nearest end when the end point is at the farthest end.
FIG. 3 shows how the position of the focus lens 20 is reset before the exposure, and how the focal point on the imaging device plane (image-space distance) is shifted during the exposure. It is noted that the shift speed of the focal point is controlled by a shift control instruction given to the focus lens 20, so that the focal point shifts on the imaging device plane at a constant speed. Typically, in the Gaussian lens law, the following relationship (Expression 1) holds where the distance between the object and the lens is u, the distance between the lens and the imaging device is v, and the focal length is f as shown in FIG. 4:
                              1          f                =                              1            u                    +                                    1              v                        .                                              Expression        ⁢                                  ⁢        1            
When there are two or more lenses, the principal point is regarded as the position of the lens. As an example, FIG. 5 shows the relationship between u and v when f is 18[mm]. When the focus lens 20 shifts, the image-space distance v between the principal point and the imaging device shifts. Here, the shift control instruction is given to the focus lens 20 so that the focal point shifts on the imaging device plane at a constant speed. Accordingly, the image-space distance v shifts at a constant speed. It is noted that, as shown in FIG. 5, the shift of the image-space distance v at a constant speed does not necessarily mean the shift of the object-space distance u at a constant speed. Here the object-space distance u is a distance between the front focal plane and the principal point. Moreover, the ordinate in FIG. 3 indicates the image-space distance v. In other words, it is noted that the magnitude relation reverses between (i) the exposure time and the object-space distance u and (ii) the exposure time and the image-space distance v. Specifically, the nearest end and the farthest end of the object-space distance u have the magnitude relation reversed in the image-space distance.
FIG. 2 shows a structure of an imaging apparatus 501 which shifts an imaging device during the exposure time. The imaging apparatus 501 in FIG. 2 includes the imaging device 1, the shutter 3, the shutter operation instructing unit 5, the release receiving unit 6, the exposure time determining unit 8, the image-restoration processing unit 11, the PSF storage unit 12, the image data recording unit 13, an imaging device initial position detecting unit 14, a synchronization managing unit 16, an imaging device shift control unit 17, and an imaging device position resetting unit 19. It is noted that the same constituent features between FIGS. 1 and 2 share the same numerical symbols. Thus detailed description thereof shall be omitted.
When the release receiving unit 6 receives an exposure start instruction from the user, the imaging device initial position detecting unit 14 detects the position of the imaging device 1 at that time (initial position). Once the initial position is detected, the imaging device position resetting unit 19 shifts the imaging device 1 to a predetermined end position, such as the nearest end or the farthest end. The imaging device position resetting unit 19 resets the imaging device 1, and at the same time, the exposure time determining unit 8 determines capturing parameters including a shutter speed and an f-number. As soon as the above operations end, the synchronization managing unit 16 gives an instruction to start exposure to the imaging device shift control unit 17 and the shutter operation instructing unit 5. Simultaneously, the synchronization managing unit 16 gives an instruction to the imaging device shift control unit 17 so that, within the exposure time and based on the end point of imaging device 1 reset by imaging device position resetting unit 19, the imaging device shift control unit 17 shifts the imaging device 1 (i) from the nearest end to the farthest end when the end point is at the nearest end, and (ii) from the farthest end to the nearest end when the end point is at the farthest end. It is noted that the imaging device 1 shifts at a constant speed.